JPH07110459A - Optical isolator - Google Patents

Abstract

PURPOSE:To enable easy assembly with low loss and less polarization dispersion without adjustment by inserting an element combined with a first double refractive plate, half-wave length plate and Faraday rotor and a second double refractive plate into groove parts at three points. CONSTITUTION:Optical fibers 1, 2 arranged in parallel in proximity to each other are cut at separate three points and are thereby formed with the groove parts. The first double refractive plate 5, the half-wave length plate 6 and the Faraday rotor 7 of a rotating angle 45 deg. in tight contact therewith and the second double refractive plate 8 are inserted into these groove parts. The double refractive plates 5, 8 are so constituted that the ordinary light of the incident light thereon advances rectilinearly and that the extraordinary light is axially misaligned and is emitted to another optical fiber. The half-wave length plate and the Faraday rotor 7 are adhered and integrated and are so constituted that the plane of polarization rotates 90 deg. with respect to the forward direction light transmitted as the ordinary light or extraordinary light through the double refractive plate 5 and that the plane of polarization does not rotate with respect to the backward direction light transmitted as the ordinary light or extraordinary light through the double refractive plate 8.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an optical isolator,
In particular, the present invention relates to a polarization-independent optical isolator using a polarization splitting element made of a birefringent crystal.

[0002]

2. Description of the Related Art An optical isolator usually has a large number of parts because it is constructed by combining individual bulk optical elements such as a polarizer, a lens, a Faraday rotator, a birefringent element, and a half-wave plate. There is a problem that the device becomes large and the adjustment of the mutual relation of the elements becomes complicated. On the other hand, it has been proposed to directly incorporate an optical isolator into a waveguide or an optical fiber to reduce the size of the optical isolator.

In Japanese Patent Publication No. 3-22962, one optical fiber is cut at one or more places, and a dielectric metal multilayer film polarizer, a Faraday rotator plate, etc. are directly inserted and integrated therein. A fiber integrated optical isolator is described. The present invention also utilizes this technique, but the one disclosed in the publication is polarization-dependent.

JP-A-3-171103 and JP-A-3-19
In No. 6115, a polarization separation element is inserted in an optical waveguide to separate the polarized light of incident light into two waveguides, which are guided to two waveguides, and a Faraday rotator and 1/1 inserted in these two waveguides.
A polarization-independent optical isolator is described, which is guided to a two-wave plate and then combined by a polarization separating element for combining. However, with this device, it is difficult to manufacture polarization separation elements for polarization separation and synthesis.

JP-A-4-307512 and JP-A-4-307512
In 349421, one optical fiber is embedded in a substrate, and a slit for obliquely cutting the optical fiber is formed in the substrate, and an optical isolator element (three beam splitters and a Faraday rotator, or two beam splitters) is provided there. And a polarization independent optical isolator in which a half-wave plate and a Faraday rotator are embedded. The angle of the end face of the optical fiber and the thickness of each element in the optical isolator element are determined so that the inclination of the light emitted from the fiber inclined end surface is canceled by each element and the light apparently goes straight through the optical fiber. The light reflected by the end face of each element is prevented from returning to the incident side due to the oblique cutting. Although this technique can provide an optical isolator that can be easily incorporated without adjustment, it has a drawback that the total thickness is increased and diffraction loss is increased because all the optical isolator elements are integrated.

[0006]

[Patent Document 1] Japanese Patent Application Laid-Open No. 4-30751
The techniques described in Japanese Patent Laid-Open No. 4-349421 and Japanese Patent Laid-Open No. 4-349421 have a drawback that the loss becomes large.

[0007]

According to the present invention, three parallel groove portions are formed in the middle of two parallel linear waveguides or optical fibers, and the first birefringent plate, 1 / An element combining a two-wave plate and a Faraday rotator and a second birefringent plate are inserted so that light in the forward direction enters from one waveguide or an optical fiber without depending on polarization, and the other waveguide or Provided is a polarization-independent optical isolator configured to be emitted to an optical fiber and not to return light to the one waveguide or the optical fiber. According to this configuration, since the birefringent plates are separately inserted into the slits, the loss is low, the assembly can be easily performed without adjustment, and the light is emitted from one optical fiber to the other optical fiber.
Since the optical path lengths of the two polarizations can be made equal, polarization dispersion (a phenomenon that a phase difference depending on the polarization occurs) decreases.

In a more preferable configuration, the two birefringent plates, the half-wave plate and the Faraday rotator are arranged obliquely with respect to the waveguide or the optical fiber, and the light incident on one of the waveguides or the optical fiber is arranged. Means that when the light passes through the first birefringent plate, the extraordinary light travels straight, the ordinary ray shifts its axis to the other waveguide or optical fiber, and the light passes through the second birefringent plate. In addition, it is characterized in that the extraordinary light travels straight, the ordinary light is decentered, and all the light travels to the other waveguide or optical fiber. According to this structure, the light reflected by the surfaces of the birefringent plate, the half-wave plate, and the Faraday rotator does not return to the incident direction in the direction away from the waveguide or the optical fiber, so that the return loss becomes large. can get.

[0009]

The basic principle of the present invention will be described with reference to FIG.
The optical fibers 1 and 2 each having the cores 3 and 4 are arranged in close proximity to each other. Optical fibers 1 and 2 are separate 3
Grooves are formed by cutting in places, and the first grooves are formed in the grooves.
Birefringent plate 5, half-wave plate 6, Faraday rotator 7 having a rotation angle of 45 ° and a second birefringent plate 8 closely attached to the birefringent plate 5.
Has been inserted. The birefringent plates 5 and 8 are configured such that ordinary light of the incident light travels straight and extraordinary light is off-axis and is emitted to another optical fiber. Instead of this, as shown in the following embodiments, the extraordinary light of the incident light may go straight, and the ordinary light may be off-axis and emitted to the other optical fiber. In this case, the birefringent plate 5,
It is necessary to incline the incident surface of No. 8 at an angle that satisfies the conditions. The half-wave plate 6 and the Faraday rotator 7 are bonded and integrated, and the plane of polarization is rotated by 90 degrees with respect to the birefringent plate 5 and light in the forward direction transmitted by ordinary or extraordinary light.
The plane of polarization is configured not to rotate with respect to light in the opposite direction that has passed through the birefringent plate 8 as ordinary light or extraordinary light.

The operation of the optical isolator of the present invention will be described. As shown in FIG. 1A, for the light in the forward direction, the light incident from the optical fiber 1 is divided into two by the first birefringent plate 5. For example, ordinary light goes straight on and goes along the optical fiber 1 for 1/2.
The plane of polarization passes through the wave plate 6 and the Faraday rotator 7 and is 90
The light, which has been rotated by a degree and becomes an extraordinary light, causes an axis shift in the second birefringent plate 8 and is emitted to the optical fiber 2. On the other hand, the extraordinary light emitted from the first birefringent plate 5 travels through the optical fiber 2 and becomes 1/2
The plane of polarization passes through the wave plate 6 and the Faraday rotator 7 and is 90
The light that has been rotated by a degree and becomes ordinary light travels straight through the second birefringent plate 8 and is emitted to the optical fiber 2. Thus, according to the present invention, a polarization-independent optical isolator can be obtained. Further, since the optical path difference between the two polarized lights is 0, there is no phase polarization dispersion.

As shown in FIG. 1B, with respect to the light in the opposite direction, the return light from the optical fiber 2 is 2 by the second birefringent plate 8.
Ordinary light travels straight through the optical fiber 2 and passes through the Faraday rotator 7 and the half-wave plate 6 and is still ordinary, and travels straight through the optical fiber 2 through the second birefringent plate 8. . On the other hand, the extraordinary light emitted from the second birefringent plate 8 remains as extraordinary light even if it travels through the optical fiber 2 and passes through the Faraday rotator 7 and the half-wave plate 6. 5
The optical axis 1 is misaligned and the light is not emitted to the optical fiber 1.

Next, the reason why the optical isolator of the present invention has a low loss will be described. The diffraction loss L (unit: dB) that occurs when a groove is formed in the waveguide or the optical fiber is the groove width d.
As an approximate expression when is not large

## EQU1 ## L = 0.11λ ² d ² / w ⁴ (Equation 1) λ is the wavelength of light, w is the effective spot size of the waveguide or optical fiber or the spot size (radius) of the optical fiber, d is the effective groove width, and the groove width is divided by the refractive index of the material to be inserted. It is a value. Japanese Patent Laid-Open No. 4-307
In the conventional example shown in Japanese Patent Application Laid-Open No. 512-512, and Japanese Patent Application Laid-Open No. 4-349421, the first birefringent plate, the half-wave plate, the Faraday rotator, and the second birefringent plate are integrated. The indicated d becomes large and the loss increases. On the other hand, in the present invention, since two optical fibers or waveguides are used and the elements are separately inserted, the loss can be reduced. For example, the first birefringent plate and the second birefringent plate are rutile plates each having a thickness of 850 μm, and the Faraday rotator is a Bi-substituted garnet having a thickness of 350 μm.
The wave plate is a quartz plate with a thickness of 92 μm, w = 20 μm, λ =
In the case of 1.55 μm, the loss L = 1.4 in the above conventional example.
However, in the present invention, the loss can be greatly reduced to L = 0.5 dB.

[0013]

【Example】

Embodiment 1 FIG. 2 shows a preferred embodiment of the present invention. This example is different from FIG. 1 in that the optical fibers 1 and 2 are formed with slanting grooves, and the first birefringent plate 15 and the half-wave plate 16 are formed in these grooves.
Faraday rotator 1 with a rotation angle of 45 °
7 and the second birefringent plate 18 is inserted. Here, the oblique angle is set so that the extraordinary light travels straight through the birefringent plates 15 and 18. At this time, the ordinary light is shifted in axis and enters the other optical fiber. Further, since the birefringent plate is inclined, the reflected light from the surface of the birefringent plate does not return to the optical fiber on the incident side, so that the return loss can be increased. As an example, the refractive index of the optical fiber is 1.45, a rutile parallel plate is used as a birefringent plate, and the C axis is arranged at an angle of 45 degrees with respect to the entrance / exit surface of the rutile plate, and the refractive index of ordinary light is set. 2.44, refractive index of extraordinary light
If the incident angle is 69, the extraordinary light goes straight as shown in FIG. 2 when the incident angle is 12.3 °.

Example 2 FIG. 3 is a schematic diagram of the optical fiber of Example 1 shown in FIG.
The optical waveguides 1 and 2 parallel to the substrate 20 are formed by using thin film technology, and groove portions for obliquely cutting these waveguides are provided at three places, and the groove portions satisfy the same relationship as that of the first embodiment. First birefringent plate 15 and half-wave plate 1 having a thickness
6, the Faraday rotator 17 having a rotation angle of 45 ° and the second birefringent plate 18 which are in close contact with this are inserted. The optical isolator of this example functions similarly to the first embodiment.

In the operation of the optical isolators of the first and second embodiments, out of the light in the forward direction incident on the optical fiber 1 or the optical waveguide 1, the extraordinary light travels straight through the first birefringent plate 15 and then has a half wavelength. After passing through the plate 16 and the Faraday rotator 17, the light becomes ordinary light, and finally the second birefringent plate 18 shifts the axis to emit the light to the optical fiber 2 or the waveguide 2. On the other hand, optical fiber 1
Or, of the light in the forward direction incident on the waveguide 1, the ordinary light is the first light.
Is guided to the optical fiber 2 or the optical waveguide 2 with its axis deviated by the birefringent plate 15, and then passes through the half-wave plate 16 and the Faraday rotator 17 to become extraordinary light, and finally, the second birefringent plate 1
8 goes straight and is emitted to the optical fiber 2 or the optical waveguide 2. Since each surface of the birefringent plate is inclined with respect to the waveguide or the optical fiber, the reflected light on these surfaces does not return to the optical fiber 1 or the waveguide 1 on the incident side. With respect to the light in the opposite direction, the return light from the optical fiber 2 is split into two by the second birefringent plate 18, and the extraordinary light goes straight to the optical fiber 2
Is an extraordinary ray even after passing through the Faraday rotator 7 and the half-wave plate 6, and goes straight through the optical fiber 2 or the waveguide 2 via the first birefringent plate 15. On the other hand, the ordinary light that has become off-axis in the second birefringent plate 8 remains ordinary light even after traveling through the optical fiber 1 or the waveguide 1 and passing through the half-wave plate 6 and the Faraday rotator 7. The birefringence plate 15 of No. 1 causes an axis shift and does not emit to the optical fiber 1 or the waveguide 1.

[0016]

According to the present invention, a polarization-independent optical isolator that is integrated with an optical fiber or an optical waveguide and can be easily assembled without adjustment is obtained. Further, in the present invention, two fibers or two optical waveguides are used, and further, different elements are inserted in different groove portions, so that light diffraction loss is small. Moreover, since the optical path difference between the two polarized waves is 0, the phase polarization dispersion is small. Further, by arranging the elements obliquely, the return loss can be increased.

[Brief description of drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is a diagram showing an optical isolator according to a first embodiment of the present invention.

FIG. 3 is a diagram showing an optical isolator according to a second embodiment of the present invention.

[Explanation of symbols]

 1, 2: Waveguide (optical fiber or optical waveguide) 5, 15: First birefringent plate 6, 16: 1/2 wavelength plate 7, 17: Faraday rotator 8, 18: Second birefringent plate 20 :substrate

Claims (2)

[Claims] 1. Three parallel groove portions are formed in the middle of two parallel linear waveguides or optical fibers, and the first groove is formed in these grooves.
Birefringent plate, an element that combines a half-wave plate and a Faraday rotator, and a second birefringent plate are inserted, and light in the forward direction is incident from one waveguide or optical fiber without depending on polarization. A polarization-independent optical isolator configured so as to be emitted to the other waveguide or the optical fiber, and return light not to be returned to the one waveguide or the optical fiber. 2. Two birefringent plates, a half-wave plate, and a Faraday rotator are arranged obliquely with respect to a waveguide or an optical fiber, and the light incident on one of the waveguides or the optical fiber is When the light passes through the first birefringent plate, the extraordinary light goes straight, and the ordinary light shifts its axis to the other waveguide or optical fiber, and when the light passes through the second birefringent plate, the abnormal light goes out. 2. The optical isolator according to claim 1, wherein the light travels straight and the ordinary light is off-axis so that all the light travels to the other waveguide or optical fiber.